Swedish cheese is made from pasteurized cows milk which is fermented by a starter culture of lactic acid bacteria. The acidified milk is curdled by rennet (chymosin) and the coagulated milk is cut and stirred. The mixture of whey and cheese grains is gently heated. The whey is separated and the cheese grains pressed to a cheese which is salted and ripened; the order of whey separation and pressing depends on the cheese variety. During the ripening a secondary flora of mainly lactic acid bacteria is growing spontaneously.
Swedish hard and semi-hard cheese will during the ripening be dominated by a spontaneously growing secondary microflora, often referred to as non-starter lactic acid bacteria, NSLAB. This spontaneous flora succeed the added starter culture and grow under the selective conditions of a maturing cheese. NSLAB are thought to enter the dairy plant either with the raw milk after surviving the pasteurization or with other ingredients used for the cheese making. The NSLAB most commonly found in Swedish and Norwegian cheese belong to the genus Lactobacillus and especially the species Lactobacillus paracasei, see Lindberg, A.-M., et al., Bacterial flora of Norwegian and Swedish semi-hard cheese after ripening, with special reference to Lactobacillus, Netherlands Milk & Dairy Journal 50 (1996) 563-572. NSLAB start to grow after a few days of ripening and reach levels of about 106-107 cfu/g after one month of ripening and this level is maintained for at least five months. Cheddar cheese show the same development of NSLAB dominated by lactobacilli. Examples of species reported from Cheddar are Lactobacillus casei, Lactobacillus plantarum and Lactobacillus brevis, but usually the dominating species are Lactobacillus casei or Lactobacillus paracasei. As the NSLAB is not controlled, it is plausible that some of the variations in cheese quality is due to the variability in the composition of the NSLAB. In order to control the process of ripening and the growth of the spontaneous flora of NSLAB, pure cultures of strains of for instance Lactobacillus have been used as adjuncts in cheese manufacturing. Said adjuncts in general might have an effect on the aroma and flavour of the cheese product; an effect which is not predictable but has to be tested by trial and error.
Probiotic microorganisms in dairy products have been the subject for intense research during the last decade. The potential health-promoting effects of dairy products which incorporate probiotic organisms, such as Lactobacillus and Bifidobacterium spp., has stimulated said research. Probiotic bacteria are described as “a live microbial feed supplement which beneficially affects the host animal by improving its microbial balance” which upon digestion in certain numbers exert health benefits, Fuller, R., Probiotics in man and animal, Journal of applied Bacteriology, 66 (1989) 365-378. Desirable traits for selection of functional probiotics are summarized and described by Klaenhammer, T. R., et al., Selection and design of probiotics, International Journal of Food Microbiology 50 (1999) 45-47. The selection criteria are said to fall into four basic categories, that is appropriateness such as nontoxicity, technological suitability such as viability, competiveness that is capability to survive in the gut, performance and functionality. Understanding the mechanisms of how these criteria impact in vivo functionality will present a major challenge in the future.